1. Field of the Invention
This invention relates in general to data storage systems, and more particularly, to a method and apparatus for controlling write operations to a data storage medium in response to the data storage system being subjected to a shock event.
2. Description of Related Art
A typical data storage system includes a magnetic medium for storing data in magnetic form and a transducer used to read and/or write magnetic data from/to the storage medium. A disk storage device, for example, includes one or more data storage disks coaxially mounted on a hub of a spindle motor. The spindle motor rotates the disks at speeds typically on the order of several thousand revolutions-per-minute. Digital information, representing various types of data, is typically written to and read from the data storage disks by one or more transducers, or read/write heads, which are mounted to an actuator assembly and passed over the surface of the rapidly rotating disks. In a typical digital data storage system, digital data is stored in the form of magnetic transitions on a series of concentric, spaced tracks comprising the surface of the magnetizable rigid data storage disks. The tracks are generally divided into a plurality of sectors, with each sector comprising a number of information fields. One type of information field is typically designated for storing data, while other fields contain track and sector position identifications and synchronization information, for example. Data is transferred to, and retrieved from, specified track and sector locations by the transducers, which follow a given track and move from track to track, typically under the servo control of a controller.
Writing data to a data storage disk generally involves passing a current through the write element of the transducer assembly to produce magnetic lines of flux, which magnetize a specific location of the disk surface. Reading data from a specified disk location is typically accomplished by a read element of the transducer assembly sensing the magnetic field or flux lines emanating from the magnetized locations of the disk. As the read element passes over the rotating disk surface, the interaction between the read element and the magnetized locations on the disk surface results in the production of electrical signals in the read element. The electrical signals correspond to transitions in the magnetic field.
To reduce system errors, it is desirable to locate the read/write elements within the boundaries of each track during the read and write operations of the disk drive. If the read/write elements are moved toward an adjacent track by an external disturbance, the data in the adjacent track can be corrupted if a write operation is in progress. For example, if the read/write transducers move while the system is writing, the new data may write over the old data on the adjacent track, resulting in an unrecoverable loss of the old data.
Present data storage systems typically prevent head movement by employing a closed-loop servo control system. During normal data storage system operation, a servo transducer, generally mounted proximate the read/write transducers, or, alternatively, incorporated as the read element of the transducer, is typically employed to read information for the purpose of following a specified track (track following) and seeking specified track and data sector locations on the disk (track seeking).
Despite the servo system, data storage systems are susceptible to problems arising from external shock and vibrational loads. An excessive shock or vibrational load (shock event) may cause the read/write elements to move off track, for example, to an adjacent track. If this head movement occurs while the drive is writing data, the old data on the adjacent track may be lost. It is therefore desirable to have a data storage system, which prevents data from being lost when the system is subjected to a shock event. Typically servo systems are too slow to prevent at least some data from being lost, particularly if a high frequency shock event were to occur.
Typically systems for preventing write operations when the data storage system is subject to a shock event only inhibit write operations in the presence of the shock event. Oscillations in data storage systems caused by transient shock motion resulting from the excitation of the frequency component modes of the data storage system are not accounted for. That is, when the shock event stops, these systems allow write operations to be performed while post-shock motion or oscillations occur.
For example, if the initial offtrack magnitude of the read/write elements caused by a shock event is sufficiently large to be of concern, the data storage system will cause write operations to stop by setting a write inhibit flag. The write inhibit flag is then dropped when the read/write elements are positioned ontrack by the servo system. The read/write elements however are typically positioned ontrack prior to the dissipation of the energy of the shock event. In other words, the read/write elements often oscillate about the track several times before the energy of the shock dissipates. The offtrack that occurs during these oscillations is typically much larger than the initial offtrack because of the gains of the modes that are excited. If the read/write elements then move offtrack again because one or more component modes were excited by the shock, the written data may be unreadable.
It is also possible that data on an adjacent track can be overwritten and made unreadable. This can cause the data to be written away from track center, leading to damage to an adjacent track or a failure to overwrite old information. Both these events can cause unrecoverable corruption of data. Once way to ameliorate this problem is to have a high servo sample rate. But the size of the shock that can be tolerated is limited by the amount of real estate that can be devoted to the servo pattern, i.e., for any sample rate there is a large enough shock to cause off track writes.
To improve write operations during shock events, a shock sensor is often used to disable the write gate. However, this isn""t a complete solution. For example, as suggested above, the worst motion caused by the shock can arise from the dynamical response that persists long after the shock itself has ended. A requalification by the servo may be forced when a shock is detected so that the write gate is re-enabled only after the requalification. Unfortunately, a requalification process is very time consuming, e.g., taking tens of milliseconds typically. Thus, invoking a requalification process may impact system throughput.
It can be seen then that there is a need for a method and apparatus for preventing write operations during shock events of different magnitudes while maximizing the system throughput.
To overcome the limitations in the prior art described above, and to overcome other limitations that will become apparent upon reading and understanding the present specification, the present invention discloses a method and apparatus for controlling write operations for a data storage system during and after a shock event.
The present invention solves the above-described problems by providing a shock sensor that measures the magnitude of a shock event and compares the magnitude of the shock event to at least two predetermined thresholds. Write operations are then inhibited based upon the comparison of the magnitude of the shock event and the at least two predetermined thresholds.
A method and apparatus in accordance with the principles of the present invention includes detecting and measuring a shock event, determining whether the measured shock event meets a first predetermined criteria and disabling the write until the write is requalified when the measured shock event meets the first predetermined criteria.
Other embodiments of a method and apparatus in accordance with the principles of the invention may include alternative or optional additional aspects. One such aspect of the present invention is that the method further includes determining whether the measured shock event meets a second predetermined criteria, executing the write when the measured shock event does not meet the second predetermined criteria and pausing the write for a predetermined time period when the measured shock event meets the second predetermined criteria but does not meet the first criteria.
Another aspect of the present invention is that the pausing the write for a predetermined time period comprises activating an unlatched logic circuit for controlling a write gate.
Another aspect of the present invention is that the second predetermined criteria comprises a maximum threshold.
Another aspect of the present invention is that the disabling the write until the write is requalified the determining step comprises activating a latched logic circuit for controlling a write gate.
Another aspect of the present invention is that the first predetermined criteria comprises a minimum threshold.
These and various other features of novelty as well as advantages that characterize the invention are pointed out with particularity in the claims annexed hereto and form a part hereof. However, for a better understanding of the invention, its advantages, and the objects obtained by its use, reference should be made to the drawings which form a further part hereof, and to accompanying descriptive matter, in which there are illustrated and described specific examples of an apparatus in accordance with the invention.